Spot welding /certified fixed orthodontic courses by Indian dental academy


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Spot welding /certified fixed orthodontic courses by Indian dental academy

  1. 1. INDIAN DENTAL ACADEMY Development of a controller to control the strength of spot welds Leader in continuing dental education
  2. 2. Project Synopsis  Spot welding is a metal joining process which joins two or more metal sheets by applying heat and pressure. The main parameters of a spot welding process are current, force and time.  Current has been the only control parameter for weld nugget growth for years while the other parameters were maintained constant throughout the welding period. The project looks into introducing force as an additional welding parameter.
  3. 3. Project Synopsis ( cont…. 1 )  The project will investigate the ability to control weld growth at real time and maintain consistency in weld strength by integrating both the current and force control parameters.  The spot welding machine that will be used in this project will be converted from the usual pneumatic based to servo based to achieve a better control of the welding force. Electrode wear and expulsion are two phenomenon which will lead to inconsistency in weld strength.
  4. 4. Project Synopsis ( cont…. 2 )  Both these phenomena were found to occur due to inability to control force during the welding process.  The controller that will be developed in this project, which will be integrating both current and force control, will be used to study these phenomenon.  Finally the developed controller will also be used to study the ability to produced stronger welds when welding dissimilar metals and metals of different thickness.
  5. 5. Spot Welding (Introduction)
  6. 6. Introduction  Spot welding is a process in which contacting metal surfaces are joined by the heat obtained from resistance to electric current flow.  Work-pieces are held together under pressure exerted by electrodes.   Typically the sheets are in the 0.5-3.0 mm thickness range. The process uses two shaped copper alloy electrodes to concentrate welding current into a small "spot" and to simultaneously clamp the sheets together.  Forcing a large current through the spot will melt the metal and form the weld.
  7. 7. Introduction ( cont…. 1 )  Attractive feature of spot welding is a lot of energy can be delivered to the spot in a very short time (ten to one hundred milliseconds).  Permits the welding to occur without excessive heating to the rest of the sheet.  Amount of heat (energy) delivered to the spot is determined by the resistance between the electrodes and the amplitude and duration of the current.  Amount of energy is chosen to match the sheet's material properties, its thickness, and type of electrodes.
  8. 8. Introduction ( cont…. 2 )  Applying too little energy won't melt the metal or will make a poor weld.  Applying too much energy will melt too much metal and make a hole rather than a weld.  Another attractive feature of spot welding is the energy delivered to the spot can be controlled to produce reliable welds.  Much higher thermal conductivity and electrical conductivity mean that up to three times higher welding currents are needed.
  9. 9. Spot Welding (Processing)
  10. 10. Processing  Spot welding involves three stages; the first of which involves the electrodes applying being brought to the surface of the metal and applying a slight amount of pressure.  The current from the electrodes is then applied briefly after which the current is removed but the electrodes remain in place in order for the material to cool.  Weld times range from 0.01 sec to 0.63 sec depending on the thickness of the metal, the electrode force and the diameter of the electrodes themselves.
  11. 11. Spot Welding (Equipment)
  12. 12. Equipment
  13. 13. Equipment  The equipment used in the spot welding process consists of tool holders and electrodes.  The tool holders function as a mechanism to hold the electrodes firmly in place and also support water hoses which cool the electrodes during welding.  Tool holding methods include a paddle-type, light duty, universal, and regular offset. The electrodes generally are made of a low resistance alloy, usually copper, and are designed in many different shapes and sizes depending on the application needed.
  14. 14. Equipment ( cont…. 1 )  The two materials being welded together are known as the work-pieces and must conduct electricity.  The width of the work-pieces is limited by the throat length of the welding apparatus and ranges typically from 5 to 50 inches.   Work-piece thickness can range from 0.008in. to 1.25in. After the current is removed from the work-piece, it is cooled via the coolant holes in the center of the electrodes.  Both water and a brine solution may be used as coolants in spot welding mechanisms.
  15. 15. Spot Welding (Tool Style)
  16. 16. Tool Style  Electrodes used in spot welding can vary greatly with different applications.   Each tool style has a different purpose. Radius style electrodes are used for high heat applications, electrodes with a truncated tip for high pressure, eccentric electrodes for welding corners, offset eccentric tips for reaching into corners and small spaces, and finally offset truncated for reaching into the work-piece itself.
  17. 17. Spot Welding (Effects)
  18. 18. Effects  The spot welding process tends to harden the material, cause it to warp, reduce the materials fatigue strength, and may stretch the material as well as anneal it.  The physical effects of spot welding include internal cracking, surface cracks and a bad appearance.  While the chemical properties effected include the metals internal resistance and its corrosive properties.
  19. 19. Spot Welding ( Time Calculation )
  20. 20. Time Calculation  Spot welding setup can be calculated using the following equation:  Total weld time = (L + W + U) x N  Where L is the load time in seconds, N is the number of welds, U is the unload time in seconds, and W is the weld time in seconds.
  21. 21. Spot Welding ( Electrical notes )
  22. 22. Electrical notes  The basic spot welder consists of a power supply, an energy storage unit (e.g., a capacitor bank), a switch, a welding transformer, and the welding electrodes. The energy storage element allows the welder to deliver high instantaneous power levels. If the power demands are not high, then the energy storage element isn't needed. The switch causes the stored energy to be dumped into the welding transformer. The welding transformer steps down the voltage and steps up the current.
  23. 23. Electrical notes ( cont…. 1 )  An important feature of the transformer is it reduces the current level that the switch must handle. The welding electrodes are part of the transformer's secondary circuit. There is also a control box that manages the switch and may monitor the welding electrode voltage or current.  The resistance presented to the welder is complicated. There is the resistance of secondary winding, the cables, and the welding electrodes. There is also the contact resistance between the welding electrodes and the work-piece. There is the resistance of the work-pieces, and the contact resistance between the work-pieces.
  24. 24. Electrical notes ( cont…. 2 )  At the beginning of the weld, the contact resistances are usually high, so most of the initial energy will be dissipated there. That heat and the clamping force will soften and smooth out the material at the electrode-material interface and make better contact (that is, lower the contact resistance). Consequently, more electrical energy will go into the workpiece and the junction resistance of the two work-pieces. As electrical energy is delivered to the weld and causes the temperature to rise, the electrodes and the work-piece are conducting that heat away. The goal is to apply enough energy so that a portion of material within the spot melts without having the entire spot melt.
  25. 25. Electrical notes ( cont…. 3 )  The perimeter of the spot will conduct away a lot of heat and keep the perimeter at a lower temperature. The interior of the spot has less heat conducted away, so it melts first. If the welding current is applied too long, the entire spot melts, the material runs out or otherwise fails, and the "weld" becomes a hole.  The voltage needed for welding depends on the resistance of the material to be welded, the sheet thickness and desired size of the nugget. When welding a common combination like 1.0 + 1.0 mm sheet steel, the voltage between the electrodes is only about 1.5 V at the start of the weld but can fall as low as 1 V at the end of the weld.
  26. 26. Electrical notes ( cont…. 4 )  This decrease in voltage results from the reduction in resistance caused by the work-piece melting. The open circuit voltage from the transformer is higher than this, typically in the 5-10 V range, but there is a large voltage drop in the electrodes and secondary side of the transformer when the circuit is closed.  The resistance of the weld spot changes as it flows and liquefies. Modern welding equipment can monitor and adjust the weld in real-time to ensure a consistent weld. The equipment may seek to control different variables during the weld, such as current, voltage, power, or energy.
  27. 27. Electrical notes ( cont…. 5 )  Projection welding is a modification of spot welding. In this process the weld is localized by means of raised sections, or projections, on one or both of the work-pieces to be joined. heat is concentrated at the projections, which permits the welding of heavier sections or the closer spacing of welds. The projections can also serve as a means of positioning the work-pieces. Projection welding is often used to weld studs, nuts, and other screw machine parts to metal plate. It's also frequently used to join crossed wires and bars. This is another high-production process, and multiple projection welds can be arranged by suitable designing and jigging.
  28. 28. Electrical notes ( cont…. 6 )  A spot welding machine normally consists of tool and electrodes, which are mechanisms for making and holding contact at the weld. Tool holders have two functions: to hold the electrode firmly in place and to support water hoses that provide cooling of the electrodes. Weld currents can range from 4000 to 24,000 amps for different types of 1010 mild steel.
  29. 29. Circuit Diagram of Spot Welding Machine (TC 15/20)
  30. 30. Spot Welding ( Safety Factors )
  31. 31. Safety Factors  Spot welding can be extremely dangerous with the use of large amounts of current and heat. Always be sure to clamp the electrodes tightly, protect your eyes from the intense light given off in the welding process and protect your hands because the work-piece can get very hot during welding. Take all safety precautions necessary before, during and after spot welding.
  32. 32. Spot Welding ( Precise Measurement )
  33. 33. Signature Image Processing  Signature Image Processing (SIP) is a technology for analyzing electrical data collected from welding processes—usually automated, robotic welding. In developed countries, some form of welding is used in more than 50% of manufactured products. Acceptable welding requires fine tuning and exact conditions; even minute variations in conditions and control can cause a weld to be unacceptable. Thus, there has been a need for a robust, reliable, real-time welding fault detection, especially in safety-critical applications in automotive manufacture. SIP allows welding faults to be identified in real time, measures the stability of welding processes and enables welding processes to be optimized.
  34. 34. Signature Image Processing ( 1 )  The advance was possible only through the advent of more powerful PCs. The quality monitoring of automatic welding can save production downtime, reduce the need for product reworking and recall, and is becoming increasingly important as car makers increase the number of robots and reduce the number of human operators in their plants. The technology is used by eight auto component manufacturers, including leading companies in Australia and the EU, in which it has transformed welding practices, significantly improving the industrial process and the quality of the products.
  35. 35. Signature Image Processing ( 2 )
  36. 36. Resistance Welding ( An example )
  37. 37. Resistance Welding      Resistance welding is the science of joining two or more metal parts together in a localized area by the application of heat and pressure. The heat is produced by the resistance of the material to carry a high amperage current. The greater the path of resistance is, the higher the heat intensity. This heat is controlled via time application and level of current applied The pressure is applied to forge the joint and consolidate the nugget to provide the weld strength. No extraneous materials such as rods, fluxes, inert gasses, oxygen, or acetylene are required
  38. 38. Welding Machine
  39. 39. Design philosophy of machinery     Interchangeable top and bottom electrode systems providing infinitely variable adjustment for all welding configurations, both spot and projection welding with wide gap adjustment Rigid main frame of fabricated steel construction, for minimum on-load deflection Precision double or triple acting pneumatic cylinders for low inertia fast follow-up Efficient secondary loop and welding transformer for high welding currents provide low mains demand
  40. 40. Standard Machine Ranges   The standard machines have common features across the range and are manufactured in four different frame sizes. Within these frame sizes, the options of spot, projection, seam, and special application are also available. Dependant upon application, the guards for the machinery are chosen. Standard units include: gravity drop safety unit, cantilever pull-down, total enclosure or light guard protection. Seam welders come in both circumferential and longitudinal configurations and will be selected dependant on an application.
  42. 42. Literature Survey  Servo Mechanism  Design Consideration  Technical Aspect
  43. 43. Thank you Leader in continuing dental education